In X-ray video images of an anatomical subject, the anatomy of primary diagnostic interest may be surrounded, overlayed or underlayed by bone or other tissue which obscures or prevents visualization of the anatomy of interest. As is conventional in the art, where two images of the same region of an anatomical subject contain both obscuring structures and structures which change in shape or other characteristic from one image to the next, a subtraction of one image from the other will create a difference image. This difference image enhances the detectability and visibility of the anatomy of interest since the obscuring structures have either been removed or suppressed as a result of the subtraction.
In X-ray angiography, for example, a number of devices are presently available which perform this subtraction technique. In such systems, a fluoroscopic X-ray video image of the anatomy of interest is first obtained. The video image is digitized and the resultant digital words, corresponding to individual pixel values of the image, are stored in an image store memory. Shortly after the first video image is obtained, an X-ray opaque dye, such as an iodine compound, is injected into a blood vessel to cause the blood vessels in the area of interest to be enhanced. One or more subsequent X-ray video images are obtained and digitized in the same manner as the first video image. The digital words, or pixel values, of the subsequent image or images are stored in one or more second image store memories. By subtracting digital pixel values of the first image (a mask image) from the corresponding digital pixel values of the subsequent images, resultant difference images are obtained. These difference images can be converted to an analog video signal which drives a video monitor so that the resultant difference image may be displayed. In these difference images, the dye path is highly contrasted from the background, and most background anatomical landmarks are removed. A typical digital X-ray image subtraction system, as hereinabove described, is disclosed in U.S. Pat. No. 4,399,457.
A disadvantage and limitation of known image subtraction systems is that the important anatomical landmarks in the background are obscured to such an extent during the subtraction that it is difficult if not impossible to discern the position of an enhanced blood vessel with respect to such background structures. Also, should the background anatomy move between taking the first and second X-ray images, the background anatomy is not subtracted out but is blurred in the resultant subtracted image.
One prior art system that provides a difference image of the anatomy of interest which is displayed with the background anatomical landmarks is disclosed in U.S. Pat. No. 4,398,213. In this system, an X-ray video image is taken prior to injection of the dye and is stored as a mask image in a first image store. After injection of the dye, the digitized pixel values of the image stored in the first or mask image and the corresponding digitized pixel values of the current image are successively applied to a subtractor to obtain the difference image, as described above. The output pixels of the subtractor for each image, however, are all stored in a single second image store, i.e. these pixel values are added on top of previous pixel values so that the resultant second image includes the entire path of the dye. The image stored in the first image store (the image without dye contrast media) and the resultant image stored in the second image store are lastly added to each other and the result displayed on a video monitor. Thus, the path of the dye is shown along with anatomical landmarks.
The significant disadvantage and limitation of the prior art system described above is that the summing of the subtracted pixel values during the dye injection procedure causes an uneven dye path intensity to be developed in the resultant video image. For example, a portion of the image containing dye throughout most of the injection procedure would have a significantly larger number of additions performed to the corresponding pixel values, i.e., a greater amplification of these pixels, than the portions of the image having the dye component for a relatively short time. Thus, the enhancement of the dye portion in the resultant image varies as a function of the time the dye had remained in each area of the image. This unevenness in the enhancement of the dye portion may adversely effect the usefulness of obscure blood vessel structures or otherwise the resultant x-ray video image as a diagnostic tool.